Spirally wound gaskets known in the art generally consist of alternate layers of a thin strip of metal, such as stainless steel, and a thin strip of a soft, non-metallic filler material which together are wound in a spiral to form an annular ring. The layered edges of the strips of filler materials form sealing surfaces. Suitable filler materials include various fluorocarbon polymers, asbestos, exfoliated graphite (Grafoil.RTM.) and ceramic.
The metallic strip is typically formed with a central, generally V-shaped fold or indentation throughout its length, and to either side of the fold are substantially flat portions. The filler material assumes this shape also.
The purpose of the V-shaped fold is to establish a path along which the metal strip can deform, in a resilient or elastic manner, as the gasket is compressed between the surfaces to be sealed. Effectively, the apex of the V-shape acts as a hinge about which the legs of the V pivot. Since the filler material itself is usually soft and non-resilient, the metal strip is required to force the filler to interfere with the surfaces to be sealed.
One drawback of previously known spiral wound gaskets is that they either tend to degrade upon exposure to highly corrosive oxidizing materials, as is the case with graphite filler material; or they degrade upon exposure to elevated temperatures, such as in a fire situation, as is the case with some fluoropolymer filler materials. A spiral wound gasket having the chemical inertness of polytetrafluoroethylene and the high temperature stability of graphite for leak prevention during fire situations is presently unknown.
Due to this degradation, known PTFE spiral wound gaskets are usually not acceptable for use in "fire-safe" valves or flanges. To qualify as "fire-safe", a valve or flange is subjected to high temperature as in an open flame environment and, over a fixed period of time during the test, leakage from the valve or flange must not exceed a certain limited quantity. A major application for "fire-safe" valves or flanges is in the transport of highly corrosive liquids such as concentrated sulfuric acid, the leakage of which during a fire can be disastrous.
Another disadvantage of these gaskets is that the bolt torque on the joint being sealed must be very high in order to sufficiently compress the gasket to achieve a tight seal. As a result, installation of the gaskets is lengthy, and the gaskets are not suitable for use in certain applications where the bolts or flanges may be adversely affected by the required torque.
Thus, a need exists for a gasket which provides an effective seal from low gasket sealing stress to high gasket sealing stress, which can be used with corrosive chemicals and which provides a measure of protection in fire situations.